Welding by various means has long been used to join together two thermoplastic pieces. Multiple thermoplastic welding technologies exist including direct heat welding such as by hot plate or hot gas; electroheat welding such as by radio-frequency means, microwaves, lasers, electrofusion, etc.; and mechanical movement joining methods such as ultrasonic vibration welding. Prior to the present invention, it had been thought that polymer materials having different properties, such as a rigid piece and a flexible piece, were generally not suitable for being welded together. It had also been thought that it was difficult to weld together plasticized polyvinyl chloride with good results, especially to another type of material.
Energy directors have been used to improve thermoplastic welding. These prior art energy directors typically have a single peak, which is sometimes roughened for particular applications.